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Power Question


shake777

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It seems to me that someone should be challenging these statements but maybe its due to lack of information as was noted previously. Thanks, Nic

Go ahead and get the sub first.

As to amplifiers (or receivers), here is the wattage I once figured was needed for each separate speaker, with all channels operating (some people allow you to cut the power needs in half to feed to each of two channels, or 1/4 for 4 channels, etc., but what if a very loud sound is way off to one side, and uses mainly just one channel?) with a speaker of typical efficiency [90 dB @ 1watt (2.73v into 8 Ohms) @ 1M). You can convert these figures to match any speakers, remembering that a 3 dB change is like doubling or halving the wattage, depending on which way you are going (I don't happen to know the sensitivity of your speakers). The typical 90 dB @ 1 w @ 1M in an anechoic chamber is increased to about 93 dB by "room gain" in a 3,000 cu. ft. room (see ahead), then reduced by 3 dB for every doubling of distance (INSIDE ... it would be 6 dB for every doubling of distance OUTSIDE). So at about 13 feet away (two doublings) we lose 6 dB, lowering our previously raised figure down to 87 dB @ 1 watt. Keep in mind that all rooms are a bit different, and there will be high and low bass pressure zones in the room. Moving on up in sound pressure level (almost equivalent to "loudness"), we need 2 watts at 90 dB, 16 watts at 99 dB (pretty loud), but 128 watts at the 108dB level THX often measures in theaters (e.g., in tests while running The Empire Strikes Back -- in the deep bass, they got 110 dB, but you will have a powered sub for most or all of that range). When I've measured this very loud level in a listening room or a theater, it has been a "broad" peak, staying at 108 dB or so for anywhere from 1/2 second to 3 seconds, so I think one's amp should have a rated power of at least that 128 watt level, when using speakers of typical efficiency. To get the 115 dB peaks Paul Klipsch advocated, you would need about 341 watts for just an instant into the typical speaker (but only 63 watts into his beloved Klipschorn). Some approx 130 watt amps can squeeze out that 341, or so, if the duration is short enough. How long is an instant? Maybe 1/2 second? That's the way it looks on my meter. According to Don Keele (a former chief engineer at Klipsch) some amps can produce, without clipping, a peak of 10 dB over their rated power (by coincidence, about 10 times the wattage). He is probably talking in terms of a few milleseconds, though, and was writing back when good amp companies didn't use specs as misleading as many manufacturers of Home Theater and Car Audio do now. This would mean that an excelent 130 watt amp might produce 1,300 watts for that tiny slice of time, which into the typical speaker we are talking about, would produce about 121 dB for a split second, if the speaker didn't burn out. Keele's table lists Heritage speakers only, which are much more efficient (6 to 14 dB) than the typical speaker, but if you want to look at it, it is in Dope from Hope Vol 16, No. 1, January 1977. To be safe -- to avoid clipping that can take out your tweeters -- a more powerful amp might be a good investment, perhaps one that is rated at 300 watts or more.

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The typical 90 dB @ 1 w @ 1M in an anechoic chamber is increased to about 93 dB by "room gain" from a typical listening position in a 3,000 cu. ft. room (see ahead),

This is frequency dependent for mains and satellites and doesn't apply to signals higher than the baffle diffraction step of the cabinet. In the case of subwoofers, figuring in boundary-loading assumes the owner knows how to correctly place their equipment. It would be more conservative to stick with the lower value.

then reduced by 3 dB for every doubling of distance (INSIDE ... it would be 6 dB for every doubling of distance OUTSIDE).

Indoors, it's still 6 dB for every doubling. You can check that with a meter. Being off by 3 or more dB in calculation would be quite an issue when figuring out power demands or why things aren't as loud as they should be. [H]
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Indoors, it's still 6 dB for every doubling. You can check that with a meter. Being off by 3 or more dB in calculation would be quite an issue when figuring out power demands or why things aren't as loud as they should be. Cool

  • I agree that being off by 3 dB is quite a lot, and I also think it would be better to err on the conservative side ... for typical (90 dB/1w/1m) speakers I would feel most comfortable with amps with >300 wts rated power, but that's just my preference and quirk. Amplifiers of > 300 watts are hardly ever reviewed in the magazines, so I guess they are rather rare. .
  • I don't know what the truth of the matter is, but I've seen the 3 dB for every doubling in a room, rather than outside, in several articles[:)]. They sometimes refer to the quasi-reverberant field of a listening room radically changing the fall-off with distance.. The only one I can refer to offhand is from Paul Klipsch, somewhere in the Dope from Hope: "The Great Major Breakthrough No. 29." He measured a speaker in a room at 2 feet and 16 feet, and got an approximately 9 dB difference. He wrote "Square-law variation of sound pressure level in a free field would dictate a 6 dB reduction in level for each doubling of distance, thus for 4, 8, and 16 feet the level should have decreased 6, 12, and 18 dB. The average difference in level of the two curves [he had run a 20 to 20K frequency sweep at 2 and 16 feet and plotted the curves, one above the other on the same graph paper] is about 8 or 9 dB. Thus the sound pressure level at the listener is about 9 or 10 dB higher than it would have been in a 'free field'. This indicted that the increased level, 9 or 10 dB, or increased power at the ear, is 10 times what it would have been in free space such as outdoors."
  • I would have been surprised if it was "still 6 dB for every doubling" indoors. The walls and ceiling would be expected to have some effect on the SPL[:)].
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Thanks for the information on this "power question"

After reading these recent post, I am starting to understand the "head room" argument.

Seems to me you need to use these calculations based on speaker efficiency and room size to get a minimum and then add a boat load of power for head room to suit you budget.

This forum rocks

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The salesperson has the right idea, however even 100 to 150 watts of clean power from a real amplifier versus the so called 140 watts from your receiver can make a huge difference in sound quality. Its not just about playing loudly, its about the fact that a receiver amp cannot deliver nearly enough current to get the most out of a large speaker ,when in surround mode.

A separate amplifier will possess much higher current reserves than a receiver, thus producing much cleaner output,which opens up the soundstage. Just because a speaker is efficient doesn't mean that it should be starved of power. You should listen to the salesperson, there is a reason that your speakers are rated for what they are.

In truth your receiver only puts out approximately 30 watts per channel 20hz-20khz with all channels driven. Read the review on Home Theater magazine's site,go to the measurements page, you'll see what I am telling you is true.

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... even 100 to 150 watts of clean power from a real amplifier versus the so called 140 watts from your receiver can make a huge difference in sound quality. Its not just about playing loudly, its about the fact that a receiver amp cannot deliver nearly enough current to get the most out of a large speaker ,when in surround mode.

A separate amplifier will possess much higher current reserves than a receiver, thus producing much cleaner output,which opens up the soundstage.

I agree with this. Some people use 3 different stand-alone 2 channel power amps to get 5 Home Theater channels (one channel is left unused), or 4 for 7 channels. Others use expensive monoblocks. I think someone -- the Fedral Trade Commision?? -- should prevent manufacturers from using the two channel results to characterize the power of a 5 or 7 channel receiver. Who would buy a 5 or 7 channel unit if they were only going to use 2 of the channels?

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It only makes sense doesn't it ? A receiver is (in most cases) a series of compromises.It attempts to fit a switching matrix,preamp,processor,tuner and amplifier into a somewhat more compact chassis,while hopefully (at least in theory) making its various features and functions more easily utilized .

While convenient in many ways (at least functionally) ,the most compromise is usually in the area of amplification.As you are in agreement with me on the subject of " the value of good amplification" I also am in agreement with you when it comes to the F.T.C..I think they should introduce a stricter and more standardized way of rating surround sound receivers' amplifiers. Manufacturers should be required to post the true power output of a receiver's amplifier with all channels driven and it should be full a full bandwidth rating - from 20hz to 20khz . I believe that this will allow the consumer to make more informed decisions, while also allowing the consumer to see the value in higher quality amplification.

Also I believe that good clean amplification shouldn't be exclusive to the high end. There needs to be a better variety of affordable, high quality amplification, from recognized brands, and not just internet based companies.If I could afford it , I too would utilize separate amplification for every channel,my wife would think I was crazy but oh well I've been called worse...LOL.

By the way in reference to the extra, left over channel in the aforementioned systems, what do you think of( if so equipped) either bi-amping the center speaker ,or even run two centers ?

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I totally agree about the standardization of power output rating. That said, I still think I'm better off getting the sub I want while its available and then get the amp of my dreams when I can afford it. I know its a questionable call but that's the way I'm choosing to do it. I think I would always pine for the second sub 12 if I didn't. Nic

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I completely disagree that high power amps are harsh at low volume. I am a live sound engineer and have been freelancing as well as doing live sound for worship in small, medium and large sanctuarys for many years and have not found that to be true at all. Yes, some amps do sound a little better when driven hard, but I've never heard them sound harsh at low volume.Furthermore, I work for a retailer which sells receivers and amps which encompass the spectrum from very low to very high end. All have their own "sonic signature". In my experience it has alot to do with gain staging and proper matching. There is nothing wrong with having more power than is needed, its called "headroom" which allows the system to operate with lower stress, at vanishingly low distortion, therefore allowing the electronics and speakers to posses the ability of producing better dynamics. Yes, you should gear your amplification to the specific needs of your environment and listening habits,however you should still, even in a home environment, atleast meet the minimum requirements of your system. If your speakers are rated for 175 watts rms, ideally you should have atleast that amount available. Not for "blowing the windows out" or "waking the neighbors", but for dynamics and clarity.This will allow the system to sound smoother, more open ,and will even add more "punch".

I did try high power a while back, although it was an older amp (Carver M-500t 250W/ch). It was impressive and pretty good sounding; even at low levels you could certainly tell that the amp had a strong hold of the speakers and was quite eager to make them move!

I'm not saying it was not a good match for the La Scalas, it was kinda fun, but then I'm sure if I had left it hooked up for any length of time it would have eventually ended up testing the local police response time. ;)

I have an old Sansui from the early seventies that I put in sometimes. It's rated 28W but the power transformer and caps are the size usually found in 100W amps. It sounds pretty sweet with a lightly rolled off top end and a firm bass with no real low bass - similar to my SETs but not as amazing; still a very fine sound.

I guess I'm a SET kind of guy. As the lead guitar player in a few bands, I get my fill of the thrill of loud music, so when I listen at home I just play it easy.

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By the way in reference to the extra, left over channel in the aforementioned systems, what do you think of( if so equipped) either bi-amping the center speaker ,or even run two centers ?

Two centers -- two La Scala IIs -- would be interesting. That way I could put my Klipschorns along the long wall (25 feet). Right now, even with a Belle in the center, there are gaps in the image on that long wall because one can't get far enough away from it. This is academic, because of no $$$.

I did consider one rear channel (Heresy II to match my Heresy II side surrounds) with a mixed signal, but people tell me that a single rear gets confused with the front image.

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Wow! Belles and Klipschorns, nice set up you have there. The dual center I suggested, is more for feasible for those who have smaller center speakers. I was thinking one above the screen and one below. Can your Belle Klipsch be bi-amped? I ask because for one thing it is a pretty large speaker and would probably benefit from bi-amping. Also on some of the larger Klipsch centers you have the option of bi-amping them ,which would take up the unused channel on the amplifier if there is one.

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Yea the headroom theory i think is to do with the frequency shift because the nominated ratings on speakers are incorrect to a proportion (spl sensitivity and even frequency), most power out of a speaker system is drawn from the largest speaker or speaker with the biggest excursion.

More power is needed in the lower frequency specturum (120hz and below) because more physical movement is required to reproduce the frequencys and more power is required by the voice coil, you could have a decent receiver amp that supplys enough watts to create a decent sound level but the difference is the quality in sound and how much QAULITY power is provided.

High current amps with headroom power will provide the best type of sound due to the possibility of power being provided when hitting those lower frequencys, bigger excursion moments and heavier impedance drops.

This is just my opinion feel free to object and return arguments.

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I think a Belle can be bi-amped, but my left-over power amp channel isn't a particularly good one. With a sensitivity somewhere between 98 dB @1W @1M and 104 dB @1W @ 1M, depending on who is doing the measuring & how they are doing it, and with 150 nominal watts (probably 120 wts, so called RMS equivalent) devoted to it, would bi-amping really help?

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Hey Crown1,

Thanks for the information and opinion on running amps at low volume. The whole "head room" idea is starting to make sense.

Do you or anyone else have more information about using two center channels. One above an one below the screen. This seems Ideal to me, but I am curious is there are drawbacks.

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I did consider one rear channel (Heresy II to match my Heresy II side surrounds) with a mixed signal, but people tell me that a single rear gets confused with the front image.


I'm not sure why the front centre and rear centre would get confused, since they're receiving different program material. The front centre gets dialogue, while the rear centre gets surround effects. Quite a few forum members are running 6.1 setups.
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The dual center I suggested, is more for feasible for those who have smaller center speakers. I was thinking one above the screen and one below. Can your Belle Klipsch be bi-amped? I ask because for one thing it is a pretty large speaker and would probably benefit from bi-amping. Also on some of the larger Klipsch centers you have the option of bi-amping them ,which would take up the unused channel on the amplifier if there is one.


There is no need to use all the channels your receiver has. Since it has only one common power supply, the more channels you drive, the less power per channel you have available.

Also, each output channel is for a particular speaker position. Can your receiver be configured for two front centre channels? If not, then the "spare" channel would not carry the dialogue.
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Crown 1, You are preaching to the choir at this point. I understand the benefit of higher wattage. Now how do I afford it. The second Paradigm is popping and shutting off so maybe my salesmanager will let me return it for credit and I keep one of each sub as you had originally suggested. It seems like the best solution. I'm going off to the Islands for a week and will ponder the question, but I do appreciate the time and effort you have given me on this subject. It has been very helpful. Thanks, Nic

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I spoke to a Paradigm Tech Rep and he said he would be happy to send out two amps to 6th Ave that they could easily swap out. I'd like to keep the one. I was told that would be fine as well. That would solve my problems, satisfy the store, and would be what you suggested to begin with. Thanks, Nic

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Forgot to mention that the first sub is very noisy at rest with the crossover control set to bypass. I didn't notice it at first because its in the corner by my circuit breaker which hums and drowns it out for the most part. However you can hear the hum from the main seating positions which should not be. I also found out there is a black one available which would be best since most of my system is black. I hate to be a pest to the store but I am a good customer and will continue to be. Nic

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